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老师批语：其实清除子元素浮动还有一种利用伪元素实现, 有空可以上网查一下
1，实例演示如何消除子元素浮动造成父元素高度折叠的影响
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<style>
.box1 {
width: 300px;
border: 3px dashed blue;
background: cadetblue;
}
.box2 {
width: inherit;
height: 300px;
background: coral;
}
.box2 {
float: left;
}
</style>
<title>Document</title>
</head>
<body>
<div class="box1">
<div class="box2">子元素</div>
</div>
</body>
</html>点击 "运行实例" 按钮查看在线实例
当子元素使用了float属性时，父容器无法根据子元素高度而自适应高度，为了解决该问题，通常有以下三种做法：
1，让父元素浮动。
<style>
.box1 {
width: 300px;
border: 5px dashed red;
}
.box2 {
width: inherit;
height: 300px;
background-color: lightblue;
}
.box2 {
float: left;
}
.box1 {
float: left;
}
</style>点击 "运行实例" 按钮查看在线实例
2，在浮动元素后面加一个清楚浮动的空标签
<style>
.box1 {
width: 300px;
border: 5px dashed red;
}
.box2 {
width: inherit;
height: 300px;
background-color: lightblue;
}
.box2 {
float: left;
}
.clear {
clear: both;
}
</style>点击 "运行实例" 按钮查看在线实例
3，在父元素中清除浮动
<style>
.box1 {
width: 300px;
border: 5px dashed red;
}
.box2 {
width: inherit;
height: 300px;
background-color: lightblue;
}
.box2 {
float: left;
}
.box1 {
overflow: hidden;
}
</style>点击 "运行实例" 按钮查看在线实例
2，实例演示三列布局的实现原理( 绝对定位实现, 浮动定位实现)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<style>
.container {
margin: 0 auto;
width: 800px;
}
.header {
height: 40px;
background: khaki;
}
.footer {
height: 50px;
background: lightsalmon;
}
.main {
background: lightseagreen;
margin: 5px auto;
}
.left {
width: 200px;
min-height: 600px;
background: lightgreen;
}
.cent {
min-height: 600px;
background: lime;
}
.right {
width: 200px;
min-height: 600px;
background: lightpink;
}
/* 使用 Position 分别把图片定为在左边、中间、右边 */
.main {
position: relative;
}
.left {
position: absolute;
left: 0;
top: 0;
}
.right {
position: absolute;
right: 0;
top: 0;
}
.cent {
margin-left: 200px;
margin-right: 200px;
}
</style>
<title>Document</title>
</head>
<body>
<div class="container">
<div class="header">头部</div>
<!-- 内容区 -->
<div class="main">
<div class="left">左侧</div>
<div class="cent">内容区</div>
<div class="right">右侧</div>
</div>
<div class="footer">底部</div>
</div>
</body>
</html>点击 "运行实例" 按钮查看在线实例
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